The present invention relates generally to a method and apparatus for removing aberrations in optical surfaces and more particularly to a method and apparatus for removing complex phase aberrations in windows or other optical surfaces using broadband phase conjugation technology.
The aerodynamic performance requirements for aircraft, missiles or other sensor platforms often necessitate the use of windows that have curved surfaces which are not symmetrical. In addition, these windows often exhibit localized manufacturing errors, such as thickness and curvature variations as well as prismatic or wedge effects. These imperfections and asymmetries result in complex distortions of a scene when it is viewed through such windows. Some of the distortions may be characterized as follows: (1) localized or global shape distortioons, (2) variable or localized magnification caused by the variations in optical power of the window, (3) one-dimensional magnifications and shifts caused by unequal curvatures of the window in different directions, and (4) other distortions caused by geometrical and/or surface defects on the outside surface of the window, the inside surface of the window or both the outside and inside surfaces of the window.
Sometimes the distortion caused by these irregularities is so severe that no recognizeable image can be obtained. Therefore, errors of this magnitude may be devastating to the performance of an imaging sensor placed behind the window. Similarly, these errors may significantly effect the judgement of a pilot viewing a scene through the window or sensor image because of binocular deviations or disparity. Because of these distoritons, the design of windows on high speed platforms used in conjunction with imaging sensors has, unfortunately, often been controlled primarily by the sensor imaging requirements, rather than by the aerodynamic requirements. In other instances the problem of image distortation caused by window aberrations has been avoided by simply eliminating the window element itself and viewing the scene to be observed through a hole in the aircraft. Both of these solutions compromise the performance of the vehicle.
The process of phase conjugation, which results from multiplying a complex wavefront by its complex conjugate is a well-known technique for correcting wavefront distortion. For example, the technique is commonly used to design aspheric corrector plates or aspheric elements for optical imaging systems. Although the plates are refractive, they generally correct only the aspheric portions of the wavefront for rotationally symmetrical aberrations, unlike the aberrations of the typical window in an airborne vehicle in which the entire asymmetric wavefront must be corrected.
Phase conjugation techniques using conjugate refractive (or reflective) type elements have been used in a variety of other circumstances to correct wavefront distortion. For example, in non-linear optics, a non-linear medium is used in conjunction with four-wave laser mixing to create real-time phase conjugated (corrected) wavefronts. Conjugate wavefrons have also been used to dynamically remove the effects of atmospheric turbulence from telescopes and other optical systems. In this type of application, referred to as "adaptive optics," interferometric measurements and appropriate servo loops are used to drive flexible optical elements to dynamically realize phase conjugations, and thus wavefront correction.
As far as is known, however, broadband phase conjugation has not been used to correct complex phase aberrations in aerodynamic windows or other types of "thick" surfaces (i.e. surfaces more than about a few millimeters thick).
Phase conjugation using diffraction rather than refraction or reflection is also known in the art. An example of this is holography. In a paper by G. O. Reynolds, J. L. Zuckerman, W. A. Dyes and D. Miller entitled "Holographic Phase compensation Techniques Applied to Human Cataracts," appearing in Opt. Eng. Vol. 12 No. 1, Feb., 1973, it is disclosed that laser holography can be used to see through excised human cataacts. As set forth in that paper, an extracted cataract from a human eye (representing an asymetrically distorted optical element) was mounted in a Mach-Zehnder interferometer illuminated by a laser. A hologram was constructed of the cataract at the image plane of the interferometer by well-known holographic techniques. The hologram was then placed in the same position as recorded, and the reference beam of the interferometer was blocked such that light from the laser source was transmitted through both the cataract and the corrector hologram (its conjugate element). A target placed beyond the aberrated lens (the cataract) could then be read, thus demonstrating that holographic phase conjugation techniques can be used to view a target through a badly aberrated lens using coherent light. Holographic correction has also been performed on wavefronts created by other randomizing media, as described, for example, by H. Kogelnik in "Holographic Image Projection Through Inhomogeneous Media," Bell System Tech. J. Vol. 44, p. 2451 (1965).
As can be appreciated, the laser holographic approach used in conjunction with an extracted cataract as described above, is not satisfactory for solving the aberration problems posed by a complex window. First, this technique uses a laser and as such works only for one wavelength of light (that of the coherent illumination source). It can not be used in broadband sensor applications because the diffraction effect of the hologram varies as a function of wavelength. Second, coherent illumination of the hologram is required to obtain the conjugate of the window from the hologram. Thus, the scene desired to be viewed would also have to be laser illuminated, which would also defeat the purpose of correcting for a broadband sensor application.
A talk pertaining to broadband phase conjugation using thin flat plate aberrators and entitled Phase Conjugation With Incoherent Radiation was presented by G. O. Reynolds, D. A. Servaes and J. B. DeVelis on Oct. 19, 1982 at the 1982 annual meeting of the Optical Society of America and is summarized on page 1746 of the Journal of Optical Society of America, Dec. 1982, Volume 72.
A talk entitled, "Phase Conjugation with Partially Coherent Radiation," was presented by D. A. Servaes, J. B. DeVelis and G. O. Reynolds, at the Optical Computing Conference, M.I.T. Boston, Mass., on Apr. 7, 1983. In both of the above talks it was shown that a thin, flat glass plate randomly scratched on one side could be passively phase conjugated for use with white light (i.e. broadband radiation) by placing a nearly index matched plastic replica of that plate in the image plane of the plate formed by an imaging lens, thereby creating a substantially undistorted image of a target when viewed through the lens by another imaging system.
In both of the above instances, the conjugate replica was made by placing the random glass surface against a sheet of optical quality plastic and then transferring the aberrations to the sheet of plastic by applying heat and pressure with a thermal laminating press.
It is accordingly an object of this invention to provide a new and improved method and apparatus for correcting phase aberrations in an optical surface using phase conjugation.
It is another object of this invention to provide a method and apparatus for correcting phase aberrations in a thick non-planar, optical surface such as an optical window in an aerodynamic vehicle, in order to further improve the image quality of a sensor system or viewer positioned behind the window.